a. Field of the Invention
This invention relates to a system for determining the position of a medical device within a body. In particular, the invention relates to a system that establishes a reference origin of a position coordinate system nearer the medical device to reduce errors in position measurements and that may also simplify the system by integrating components and functions of the system.
b. Background Art
It is desirable to track the position of medical devices as they are moved within a body so that, for example, drugs and other forms of treatment are administered at the proper location and medical procedures can be completed more efficiently and safely. One conventional means to track the position of medical devices within the body is fluoroscopic imaging. Fluoroscopy is disadvantageous, however, because it subjects the patient and clinician to undesirable levels of electromagnetic radiation. As a result, medical device navigation systems have been developed to track the position of medical devices within the body. These systems typically rely on the generation of electrical or magnetic fields and the detection of induced voltages and currents on position sensors attached to the medical device and/or external to the body. The information derived from these systems is then provided to a clinician through, for example, a visual display.
One conventional medical device navigation system is offered for sale under the trademark “ENSITE NAVX™” by St. Jude Medical, Inc. The system is based on the principal that when electrical currents are passed through the thorax a voltage drop occurs across internal organs such as the heart and this voltage drop can be measured and used to determine the position of a medical device within the body. The system includes three pairs of patch electrodes that are placed on opposed surfaces of the body (e.g., chest and back, left and right sides of the thorax, and neck and leg) and form generally orthogonal x, y, and z axes as well as a reference electrode that is typically placed near the stomach and provides a reference value and acts as the origin of the coordinate system for the navigation system. Sinusoidal currents are driven through each pair of patch electrodes and voltage measurements for one or more electrodes associated with the medical device are obtained. The measured voltages are proportional to the distance of the device electrodes from the patch electrodes. The measured voltages are compared to the potential at the reference electrode and a position of the device electrodes within the coordinate system of the navigation system is determined. Referring to FIG. 1, the inventors herein have identified a simplified zero-order circuit 10 that illustrates the system for any given axis. Although the following description refers to impedances within circuit 10, only the real or resistive portion of a given impedance is used. In the illustrated circuit 10, the medical device within the body includes three electrodes represented by impedances 121, 122, 123. Amplifiers, filters and other signal processing circuitry for each electrode create impedances 141, 142, 143. The position of each electrode along the axis is determined responsive to the voltage at nodes 161, 162, 163 which, when divided by the drive current, will yield resistances. The resistance generated by the body is represented by resistance 20, denoted as Rb. The position (P) of an electrode along an axis may be obtained using the following equation (1):
  P  =                    S        f            *              R        b            *              R        i                            R        i            +              R        e            where Re is the resistance of the individual electrode (typically 100-500 ohms, but may exceed 1000 ohms with specialty electrodes), Ri is the input resistance of the amplifier (typically 200-1000 kilo ohms), Sf is a scale factor (e.g., 25 mm/ohm), and Rb is the resistance of the body along the axis (typically 5-30 ohms).
The above-described system can be used to provide a substantially accurate indication of the position of the medical device. As indicated in the above-recited formula, however, the position determination along each axis is generally proportional to the resistance Rb of the body between the anatomical region of interest and the reference electrode. This resistance Rb varies along each axis. The varying body resistance Rb contributes to a variety of position measurement errors including: (1) drift resulting from a changing position of the medical device over time relative to the original measured position; (2) shift resulting from a change to certain parameters (e.g., connection or disconnection of the medical device or movement of the reference electrode); (3) scatter resulting from variation of electrode impedances or the amplifiers and related circuitry; and (4) offset resulting from differences in impedance among the electrodes and amplifiers and related circuitry among multiple catheters. For example, in one realistic scenario the body resistance Rb between the location of the reference electrode and the anatomical region of interest may be 12 ohms with an input resistance Ri along the sense amplifier channel of 200 kilo ohms and a scale factors Sf of 25 mm/ohm. In the case of a specialty electrode having a nominal resistance Re of 2 kilo ohms, the resistance may vary within a range of +/−30%, or between 1.4 and 2.6 kilo ohms. If electrodes having 1.4 and 2.6 kilo ohms are placed at an identical location, the position of the electrode along the axis calculated using equation (1) would yield different results at 297.9 millimeters and 296.2 millimeters, respectively—a difference of 1.7 millimeters. In the case where multiple electrodes are separate by, for example, 5 millimeters on a catheter, 1 deviation of 1.7 millimeters about a mean along any of the three axes will result in a visible error. A linear catheter may be erroneously depicted with electrode locations creating a saw tooth pattern instead of a smooth pattern. As another example, the body resistance Rb may change over time due to, for example, a saline infusion received by a patient which will generally lower body resistance. If the initial body resistance Rb is again 12 ohms, a 2% reduction in Rb would change the 297.9 millimeter position location of the electrode referred to above to 291.9 millimeters—a 6 millimeter drift.
The use and placement of the reference electrode is also disadvantageous because clinicians are required to locate and place a separate electrode and facilities must likewise maintain an inventory of the electrodes. Medical device navigation systems employ special purpose electrodes, sensors and other components that are separately connected to the body surface. Oftentimes, these systems are used simultaneously with, and even cooperatively with, other systems employing different electrodes and sensors that are also connected to the body surface. For example, electrocardiography (ECG) electrodes are typically connected to the body during the same procedures in which navigation systems are used in order to monitor critical vital signs and as an input to the navigation system to compensate for motion of the heart. Similarly, magnetic navigation systems typically employ a sensor to monitor movement of the patient in order to compensate for this movement in determining the position of the medical device. The proliferation of electrodes, sensors and other components connected to the body increases the chance of error in setting up the various systems and increases the time required to prepare and complete a procedure.
The inventors herein have recognized a need for a system for determining the position of a medical device within a body that will minimize and/or eliminate one or more of the above-identified deficiencies.